There are many known types of welding-type power supplies. Welding-type power, as used herein, refers to power suitable for electric arc welding, plasma arc cutting or induction heating. Welding type system, as used herein, is a system that can provide welding type power, and can include control and power circuitry, wire feeders, and ancillary equipment. Welding-type power supply, as used herein, is a power supply that can provide welding type power.
Providing welding-type power, and designing systems to provide welding type power, provides unique challenges. Welding type systems will often be moved from one location to another, and be used with different inputs, such as single or three phase, or 115V, 230V, 460V, 575V, etc., or 50 hz or 60 hz signals. Power supplies that are designed for a single input cannot provide a consistent output across different input voltages, and components in these power supplies that operate safely at a particular input level can be damaged when operating at an alternative input level. Also, power supplies for most fields are designed for relatively steady loads. Welding, on the other hand, is a very dynamic process and numerous variables affect output current and load, such as arc length, electrode type, shield type, air currents, dirt on the work piece, puddle size, weld orientation, operator technique, and lastly the type of welding process determined to be most suitable for the application. These variables constantly change, and lead to a constantly changing and unpredictable output current and voltage. Power supplies for many fields are designed for low-power outputs. Welding-type power supplies are high power and present many problems, such as switching losses, line losses, heat damage, inductive losses, and the creation of electromagnetic interference. Accordingly, welding-type power supply designers face many unique challenges.
Additionally, welding-type power supplies or systems are often sold for one or more particular processes, such as stick, TIG, MIG, pulse, sub-arc, heating, cutting, and the maximum output power or current can be anywhere from one hundred or less amps, to five hundred or more. The maximum output of a particular welding-type system is chosen for the process and/or commercial market for which it is intended. While welding type power is a high power level, some welding type systems must provide power and/or output current than others. For example, the required output of a 300 amp stick welding system is different from the required output of a 600 amp MIG welding system.
Prior art welding type systems have typically been designed for a particular output, and the power circuitry, controller, output circuitry, etc., are designed with the maximum output power in mind. A 100 amp system might be different from a 200 amp machine, which is different from a 300 amp machine and so forth. Thus, a welding type system is often designed from the ground up. Other times, in an effort to reduce the attending engineering costs, a welding-type power supply is scaled up for a higher output by increasing switch capacities, or placing switches in parallel. However, there are limits to this sort of scaling up, and it gets ever more costly for components to tolerate ever greater currents. Both of these approaches in designing new welding type systems required extensive design, engineering, and testing, and were thus relatively expensive.
U.S. Pat. No. 6,713,721 (hereby incorporated by reference), entitled Method of Designing and Manufacturing Welding-Type Power Supplies, issued to Albrecht on Mar. 30, 2004, teaches to use a single power topology with a given output current, and then to place modules in parallel as needed to obtain a desired output current. For example, if each module produces 250 amps, and 750 amps is needed, then three parallel modules are used. While using modules in parallel as taught in U.S. Pat. No. 6,713,721 provides for increased output current, the output voltage for multiple modules is no higher than the output voltage for a single module.
One prior art welding type power supply that is well suited for portability and for receiving different input voltages is a multi-stage system with a preregulator to condition the input power and provide a stable bus, and an output circuit that converts or transforms the stable bus to a welding-type output. Examples of such welding-type systems are described in U.S. Pat. No. 7,049,546 (Thommes) and U.S. Pat. No. 6,987,242 (Geissler), and US Patent Publication 20090230941 (Vogel), all three of which are owned by the owner of this disclosure, and hereby incorporate by reference. Miller® welders with the Autoline® feature include some of the features of this prior art.
There are many types of welding type power supplies that can provide a welding type power output from an AC or DC source of power. One general category of power supply is known as a switched-mode power supply that utilizes power semiconductor switches to chop a DC source of power and convert this chopped power to a voltage and/or current suitable for welding.
One type of switched-mode power supply is commonly known in the welding industry is an inverter type power supply. An inverter type power supply chops the source of DC power and applies it to the primary of a transformer. The frequency of the chopped voltage is typically much higher than the AC line frequency (50 to 60 Hz), commonly used as a source of power. Typical switching frequencies are in the range of 20 KHz to 100 KHz. This higher frequency allows the inverter transformer to be much small than a comparable line frequency transformer. The secondary of the transformer transforms the chopped voltage to a voltage and current level suitable for welding. Typically the secondary of the transformer is connected to a rectifier and converted to DC and fed to a smoothing inductor to filter the output. This smoothed output is then used as the output of the welding type power supply. For some welding type power sources the DC output is further processed and converted to an AC welding type output such as for AC GTAW.
There are many circuit topologies that can be used for an inverter based welding type power supply. Amongst these are topologies commonly known as forward circuit, full-bridge, half-bridge, flyback, and others. The source of DC power for these types of power supplies is typically derived by rectifying a source of AC line power. An inverter type power supply may also include a pre-regulator circuit following the rectifier and preceding the inverter circuit. The pre-regulator circuit can serve the function of providing a regulated DC bus voltage to the inverter circuit that may be at a voltage level different from the raw rectified AC voltage. This pre-regulator circuit may also include a power factor control that can be used to improve the power factor of the current drawn from the AC line. One prior art welding type power supply that uses pwm control of the output is described in U.S. Pat. No. 8,455,794, hereby incorporated by reference.
FIG. 1 shows a simplified schematic for an inverter based welding type power supply consistent with those shown in U.S. Pat. Nos. 7,049,546 and 6,987,242. AC line voltage is rectified, shown with three phase AC, could alternately be single phase. Typical values for AC line voltage can range from 115VAC or lower to 600VAC. The inverter power supply may be designed for a single nominal AC line voltage or for a range of AC line voltages. The rectifier may include a filter capacitor, shown as C3, and provide an output voltage (Vrectified).
A pre-regulator may be included to provide a regulated bus voltage (Vbus) which may be regulated to a voltage greater than the peak of the rectified AC line voltage. The pre-regulator circuit may also contain a power factor correction circuit or control to improve the power factor for the current or power drawn from the AC line. FIG. 1 shows a boost converter circuit arrangement for the pre-regulator. The switching of power semi-conductor Z3 is controlled by the gate drive signal provided by the pre-regulator/inverter control. The switching of Z3 can be controlled in such a manner to provide a regulated Vbus as well as perform power factor correction.
The inverter topology shown is a half-bridge circuit with the primary of the high frequency inverter transformer, T1, connected between the center point of capacitors C1 & C2 and the junction between power semiconductor switches Z1 & Z2. Power semiconductor switches are switched on and off by a gate drive circuit which is shown as part of the inverter control. The switching frequency and ON/OFF ratio (or duty cycle, D) of the power semiconductor switches is controlled by the inverter control to provide a regulated output voltage and/or current of the welding type power supply. Z1 & Z2 alternately chop the DC bus voltage and create a high frequency AC voltage on the primary of the transformer. For the half-bridge circuit shown the bus voltage is split in half by the two capacitors, so effectively when either Z1 or Z2 is switched on, one half of Vbus is applied across the primary of the transformer. The transformer transforms the voltage to a level suitable for welding. The center tapped secondary of the transformer is connected to a diode rectifier (D2, D3) which rectifies the secondary high frequency AC voltage to create a DC output. The DC output is filtered by inductor L1 to provide a smoothed output current to a welding arc. Additional components and circuits not shown in FIG. 1 may be included such as snubbers and pre-charge circuits, EMI filters, gate drive circuits, control power supplies and various other circuits.
A current sensor (CS1) provides a feedback signal indicative of the output current (I_out). Voltage feedback is also provided to the inverter control circuit, V_out. The inverter control may also provide other functions such as monitoring thermal sensors, controlling cooling fans, receiving and sending various status and control signals to other circuits and controls such as a welding control. The weld controller shown allows the user to select and control a welding process, and may provide various signals, indicators, controls, meters, computer interfaces, etc. to allow the user to set up and configure the welding type power supply as required for a given welding process. The weld controller typically will provide a command signal to the inverter control, indicated as I_ref. This command signal may be an output current level for the power supply or may be a more complex waveform or signal dependant on the particular weld process and user inputs, voltage and current feedback signals and other conditions at the welding arc. Voltage feedback, current feedback, and other signals may be provided to the weld control.
Welding type power supplies such as shown in FIG. 1 are often designed to operate from industrial level AC power such as 230, 460 or 575VAC. As such the bus voltage Vbus may be greater than 900 Volts. This level of bus voltage may require power semiconductor switches (Z1, Z2, Z3) that have voltage ratings on the order of 1200 Volts. Circuits such as snubbers, slow voltage transition (SVT) or other circuits may be required to reduce switching losses within the power semiconductors because of the bus voltage level. In addition a series arrangement of bulk capacitors (C1,C2) may be required to attain sufficient voltage rating. These capacitors may not share the voltage perfectly and end up with a mismatch voltage level.
Welding type power supplies are often designed with components that can handle the full range of input voltage and power and provide a given welding type power output. This may not be optimum for certain applications, making the welding type power supply more complex or expensive than may be necessary.
It can be desirable to have a welding type power supply that can handle a range of inputs such as illustrated above, yet use lower voltage power semiconductors which may switch more efficiently and reduce or eliminate the need for snubbers and other circuits to reduce switching losses. It can also be desirable to maintain a well balanced sharing of voltage for bus capacitors when a series arrangement is used. It can also be desirable to provide a welding type power supply that can be readily adapted for different input voltages and power levels along with various welding outputs.
Accordingly, a welding-type system having a power topology comprised of a modular system, preferably one that can provide a desired output current greater than the output of any one module, and/or an output voltage greater than the output voltage of any one module is desired. Preferably, the system maintains the advantages of prior art portable, universal input systems, but also avoids some of the deficiencies of the prior art is desired.